Improve test coverage

Change-Id: If5b263f3b5a71463e93f1de2fc90edaef95df2b3

@@ -18,6 +18,7 @@ package org.onosproject.store.resource.impl;
 
 import com.google.common.collect.ImmutableSet;
 import org.junit.Test;
+import org.onlab.packet.VlanId;
 import org.onosproject.net.DeviceId;
 import org.onosproject.net.resource.DiscreteResource;
 import org.onosproject.net.resource.Resources;
@@ -52,6 +53,13 @@ public class EmptyDiscreteResourcesTest {
     }
 
     @Test
+    public void testValuesOf() {
+        // doesn't match any types of resources
+        assertThat(sut.valuesOf(Object.class), is(ImmutableSet.of()));
+        assertThat(sut.valuesOf(VlanId.class), is(ImmutableSet.of()));
+    }
+
+    @Test
     public void testContainsAny() {
         DiscreteResource res1 = Resources.discrete(DeviceId.deviceId("a")).resource();
         DiscreteResource res2 = Resources.discrete(DeviceId.deviceId("b")).resource();